Three to four piston rings are attached to a piston in an internal combustion engine. Two to three upper piston rings are called compression rings, which act not only to keep gas tightness between a combustion chamber and a crank room, but also to dissipate heat from the piston to a cylinder. Also, one to two lower piston rings are called oil rings, which act not only to form an oil film on an inner surface of the cylinder, but also to scrape excessive oil.
Known as a compression ring is a taper-faced piston ring having a tapered peripheral surface, which scrapes an oil attached to an inner wall of a cylinder liner or a cylinder (hereinafter referring to as “inner cylinder wall”) when a piston moves downward, and forms an oil film by an wedge effect between a tapered surface and an inner cylinder wall when a piston moves upward, depending on a tapering angle θ. With such features, it is widely used mainly as a second ring (second compression ring) or a third ring (third compression ring) for an automobile engine.
As described above, the taper-faced compression ring should have gas tightness and a sufficient oil-controlling function. To this end, it is important to keep a shape of a tapered face for a long period of time from a stage of running-in. As shown in FIG. 4, a taper-faced compression ring 20 actually has a cross section having a tapering angle θ and a ring width h1. To improve running-in, it is usually lapped in a sleeve having an inner diameter equal to a nominal diameter of the ring in a final production step, thereby forming a belt-like contact surface 4 on an outer peripheral surface 1 of the ring. Though the contact surface is formed by lapping for a long period of time without fail, depending on the circularity of the resultant ring (the circularity of the ring when charged into a truly circular sleeve having an inner diameter equal to the nominal diameter of the ring), it may have an increased contact width. In that case, surface pressure to a cylinder wall decreases, resulting in increased oil consumption. Increase in the contact width may invite increase in friction, though variable depending on the tension of the ring, resulting in lower fuel efficiency. In the production of a piston ring, it is desired that a lapped surface having a predetermined contact width L′ is surely obtained from the aspect of quality, and that the lapping time is reduced from the aspect of cost.
JP 2002-323133 A discloses the formation of the above contact surface (described as “flat surface”) in a wire in advance, to reduce working time such as lapping time. Further, it teaches a flat surface width of 0.05-0.3 mm. However, even though a wire has a flat surface in advance, a lapped surface having a predetermined contact width is not necessarily obtained surely, depending on the circularity of the resultant ring, etc. as described above, thus actually failing to reduce the lapping time.